In general, dot matrix printers can be separated into two types of printers--line printers and serial printers. Line printers include print heads that create horizontal lines of dots substantially simultaneously as paper moves vertically through the printer. A series of dot lines create a line of characters (or a design). Serial printers include a print head that is horizontally moved back and forth across the paper, either continuously or in steps. The print head of dot matrix serial printers includes a vertical column of dot printing elements. As each column position of a character position is reached, the required number of dot printing elements are actuated. A series of dot columns creates a character (or part of a design). While developed for use in print heads for dot matrix serial printers, certain aspects of the hereinafter described invention may also be useful in dot matrix line printers.
The printing elements of the print heads of both serial and line dot matrix printers are electromagnetically actuated. The electromagnetic actuation mechanisms include hammers that press the dot printing element against the print receiving medium. In some print heads, the hammers are held in a retracted position by the magnetic force created by a permanent magnet. When retracted, the hammers are stressed and, thus, store energy. When a coil mounted on a post located in the magnetic flux path of a hammer is suitably energized, the magnetic field produced by the coil counteracts the magnetic field produced by the permanent magnet resulting in the release of the hammer. The energy stored in the stressed hammer causes the printing of a dot. One example of a print head for a dot matrix serial printer that functions in this manner is described in U.S. Pat. No. 4,225,250 entitled "Segmented-Ring Magnet Print Head" by Richard E. Wagner et al. Other examples are described in U.S. Pat. Nos. 3,592,311, 3,659,238, 3,672,482, and 4,037,704.
While the small size and lack of complexity of the print head described in U.S. Pat. No. 4,225,250 made it substantially advanced over print heads of the type described in the other patents referred to in the preceding paragraph, it remains subject to improvement. For example, its heat dissipation ability is less than desirable. Furter, it is heavier than desirable. Both of these factors restrict the speed of operation of dot matrix serial printers employing such print heads. More specifically, the print head described in U.S. Pat. No. 4,225,250 includes a ring magnet mounted on a base plate. The base plate also supports a plurality of coil/post combinations mounted inside of the ring magnet. A print hammer disc including a plurality of inwardly projecting hammer arms is mounted on the permanent magnet such that a hammer arm overlies each of the coil/post combinations. The print hammer disc is covered by a face plate. As a result, the heat generating elements of the print head--the coils--are entirely enclosed. Because the coils are entirely enclosed, the heat generated by the coils is not readily dissipated. Since heat generation is directly related to the coil energy dissipated when hammers are released, heat generation is related to the speed of hammer actuation. Since excessive heat can damage and/or destroy the release coils, printer speed is related to heat generation and, thus, heat dissipation. Since the heat dissipation ability of print heads of the type described in U.S. Pat. No. 4,225,250 is limited, print speed is likewise limited. Further, the ring magnet used in such print heads is relatively large since it surrounds the coil/post combinations. Not only is the ring magnet large and, thus, heavy, the remaining ferromagnetic elements that make up the magnetic circuit of the print head--the base plate and the face plate--are large and, therefore, heavy. As will be understood by those familiar with mechanisms for moving print heads, heavier print heads require more movement energy than do lighter print heads in order to achieve the same speed of operation. In addition to increased operating expense, higher energy requirements necessitate greater cooling in order to avoid breakdowns caused by overheating.
The invention is directed to providing a print head that avoids these disadvantages. More specifically, the invention is directed to providing a print head for a dot matrix serial printer that is lightweight and dissipates the heat generated when the hammers of the print head are actuated.